Conventionally, a construction machine has been proposed that has an electric motor as a power source for a turning mechanism for turning an upper-part turning body. In such a construction machine, the turning mechanism is accelerated (driven) by the power running operation of the electric motor, while a regenerative operation is performed at the time of decelerating (braking) the turning mechanism to charge a battery with generated electric power (for example, Patent Document 1). Further, the construction machine described in Patent Document 1 has a hydraulic pump in order to hydraulically drive other drive mechanisms than the turning mechanism, and has a generator connected via a speed-increasing machine to an engine for driving this hydraulic pump, so as to use the electric power produced by power generation to charge the battery and to drive the electric motor of the turning mechanism.    [Patent Document 1] Japanese Laid-Open Patent Application No. 2004-036303